The reason would be so that you don't kill them in the day and as a group you work with the zombies, controlling the game. I do think it would have been stupid for the zombies to do that but what had you got to lose to try?Yeah... I'm sure zombies would put their entire game at risk just to tell us in the inn that one of us is infected. If there was a mafia in the same inn then it would have been game over for them but w/e right??? They can just infect people without telling them, there is no reason to say anything like that in the inn chat.
Plus if it didn't go well you would be put under slight suspicion the next day - hopefully enough not to be lynched but enough to make you not get infected at night since they would be aiming for zombies